Agricultural or Dolomite Limes (trade name 'Super Sweet') can be applied to the lawn anytime of year that it will be washed into the soil. Early spring and fall are the most efficient as rain will do the job for you. Buttercups and moss are symptoms of acid soil and lime helps to change the pH of the soil so that it is more basic and better for lawn growth. The bad news here is that lime works slowly. It may take several years of application to see the buttercups and other acid loving lawn weeds disappear. Anything you do to improve drainage will also help to get rid of buttercups as they need a moist soil. Aerate the lawn and add sandy topsoil to the area with buttercups. Keep growing, Marianne Binetti